This invention relates to high-intensity-discharge (HID) sodium lamps and, more particularly, to an improved starting arrangement for such lamps.
HID sodium lamps are relatively difficult to start and normally require the application of a very high voltage pulse across the lamp electrodes. Other types of HID lamps incorporate a starting electrode sealed through an end of the arc tube and which is closely spaced to one of the main electrodes. In the case of HID sodium lamps, however, the space limitations normally preclude such a starting electrode and if metallic end caps are used to seal off the ends of the tubular arc tube, which is normally fabricated of alumina, it is difficult to insulate the starting electrode from the proximate main electrode.
A starting electrode for an HID sodium lamp is disclosed in Japanese Pat. No. 47-49382 dated Dec. 12, 1972. As shown in FIG. 2 of this patent, the starting aid comprises a metallic, annular-shaped member which is sealed on both sides to two tubular-shaped envelope members to form the arc tube body.
In German published patent application No. 2,316,857 dated Oct. 3, 1974 is disclosed a starting electrode for HID sodium lamps wherein a metallic coating (5a in the figures) is formed on the face of a ceramic ring 5 which, in turn, is sealed to the main tubular ceramic body to form the arc tube.
U.S. Pat. No. 3,461,334 dated Aug. 12, 1969 to Knochel et al. discloses a starting electrode for an HID sodium lamp wherein an annular-shaped metallic member is sealed to two tubular-shaped ceramic members to form the composite arc tube with the sealed starting arrangement.
Japanese Preliminary Publication of Utility Model patent application 49-102573 dated Sep. 4, 1974 discloses a starting electrode which is sealed through the ceramic end cap portion of a ceramic arc tube.
Various sealing materials for sealing refractory metals to alumina are known and U.S. Pat. No. 3,469,729 dated Sep. 30, 1969 to Grekila et al. discloses a calcia-alumina-silica composition for sealing tantalum or niobium to alumina. In U.S. Pat. No. 3,480,823 dated Nov. 25, 1969 to Chen is disclosed a somewhat similar composition which incorporates from 2% to 5% by weight of niobium powder to improve the bonding strength of the seal.
The use of a thermal switch which is responsive to the heat generated by an operating lamp to remove a starting potential from a starting electrode for an HID metal-halide-type lamp is shown in U.S. Pat. No. 3,226,597 dated Dec. 28, 1965 to Green, and U.S. Pat. No. 3,746,941 dated July 17, 1973 to Olson et al. discloses an HID sodium lamp wherein a wire starting aid is coiled about the arc tube, and after the lamp is operating, bi-metal switches isolate the starting aid from other electrical elements of the lamp.